The present invention relates to a heat pipe system, and in particular, a two phase loop commonly known as a Loop Heat Pipe (LHP) or Capillary Pumped Loop (CPL).
A basic heat pipe comprises a closed or sealed envelope or a chamber containing a liquid-transporting wick and a working fluid capable of having both a liquid phase and a vapor phase within a desired range of operating temperatures. When one portion of the chamber is exposed to relatively high temperature it functions as an evaporator section. The working fluid is vaporized in the evaporator section causing a slight pressure increase forcing the vapor to a relatively lower temperature section of the chamber defined as a condenser section. The vapor is condensed in the condenser section and returned through the liquid-transporting wick to the evaporator section by capillary pumping action.
Because it operates on the principle of phase changes rather than on the principles of conduction or convection, a heat pipe is theoretically capable of transferring heat at a much higher rate than conventional heat transfer systems. Consequently, heat pipes have been utilized to cool various types of high heat-producing apparatus, such as electronic equipment (See, e.g., U.S. Pat. Nos. 5,884,693, 5,890,371, and 6,076,595).
Because conventional heat pipes must transport liquid through the capillary wick, they incur a large flow pressure drop if they are made very long. Also, because liquid and vapor flow in opposite directions, vapor can entrain liquid at high power rates and limit the operation of the device; this is commonly known as the flooding limit. To overcome these limitations and transport high thermal power over long distances, the Loop Heat Pipe (LHP) and Capillary Pumped Loop (CPL) were developed. (See notably U.S. Pat. No. 4,515,209.)
In conventional heat pipes, heat almost always enters the heat pipe from the liquid (i.e., convex) side of the meniscus. As is known in the art, the meniscus is the curved shape of the surface of a liquid in a container, caused by the cohesive effects of surface tension (capillary action).
Alternatively, in capillary pumped two phase loop heat pipes, such as loop heat pipes (LHPs) and capillary pumped loops (CPLs), heat enters the device (e.g., LHP, CPL, etc.) from the vapor (i.e., concave) side of the meniscus. This is known as an inverted meniscus arrangement.
Because of the xe2x80x98inverted meniscusxe2x80x99 arrangement, devices such as LHPs and CPLs have relatively high thermal resistance in the evaporator area, and are typically not capable of operating at high heat fluxes without drying out. Thus, conventional LHPs/CPLs can dissipate approximately only 10 W/cm2.
Some have utilized a method of filling the vapor spaces of the evaporator portion of the LHP/CPL with bidispersed wick in order to achieve higher heat dissipation figures. LHPs/CPLs with bidispersed wicks have achieved approximately 100 W/cm2 of heat dissipation, however, at the expense of constricting vapor flow and maximum power capacity, as well as introducing considerable complexity and cost.
The nature of a two phase loop requires that the temperature difference (often referred to as xe2x80x98delta Txe2x80x99 or xe2x80x98xcex94Txe2x80x99) from the vapor to the liquid side of the wick correspond to the capillary pressure being produced by the wick. If the xcex94T is insufficient, then boiling will occur on the liquid side of the wick and the loop will deprime (i.e., stop operating). This xcex94T relationship becomes increasingly more difficult to maintain as the wick dimensions are made smaller. Miniature evaporators for two phase loops are thus very difficult to design and build, and sub-miniature evaporators of a size that would permit integration with a semiconductor chip have so far not been feasible.
Therefore, there is currently a need for a two phase loop system which can accommodate high heat flux inputs without also restricting vapor flow. There is also a need for means to maintain a suitable xcex94T in evaporators scaled to permit integration with semiconductor chips.
The present invention is a evaporator including a primary wick which controls evaporation from a primary heat input area and, a secondary wick which separates liquid and vapor volumes disposed in the evaporator, and which feeds liquid to the primary wick. The present invention also includes a method for cooling heat-producing equipment by disposing a heat-producing apparatus on the convex side of a meniscus of liquid located in a wick of a loop heat pipe.
The above and other advantages and features of the present invention will be better understood from the following detailed description of the exemplary embodiments of the invention which is provided in connection with the accompanying drawings.